Previous studies have amply demonstrated that bone cartilage and dentin growth/mineralization in rats has circadian and ultradian components that are related to the activity of the parathyroid-adrenal axis. In further evaluation of the skeletal biorhythms, an attempt will be made to identify the in vivo zeitgebers that drive the rhythms (meal timing, pituitary pineal-adrenal axis) and to examine the in vitro possible fluctuations in the size of the osteoprogenitor cell- osteoblast populations in marrow. Knowledge of these parameters of skeletal growth will make it possible to better predict the effect of timing of chronopharmacological intervention.